This invention generally relates to geometry checking arts for checking the geometry of a circuit pattern of a semiconductor device formed on an LSI wafer and more particularly to a method of auto-focusing and an apparatus therefor.
Integrated circuits such as LSIs have a tendency toward high density packaging and miniaturization. In formation of fine patterns for use in such integrated circuits, it often happens that pattern defects occur even if the formation process is very carefully handled, and careful inspection is required. In primitive inspection, a number of inspectors check patterns by the eye using a microscope and their eyes get tired with a long term inspection to overlook many defects, causing problems in quality control. It is therefore desired to automate the production process but in automating the inspection process, there still remains a significant problem of focusing.
Many proposals have been made mainly in techniques of auto-focusing of cameras. For example, Japanese Patent Unexamined Publication No. 58-91409 discloses an arrangement to be described below.
More particularly, a light beam emitted from a light source is irradiated on a circuit pattern formed on a wafer, a resulting image of the circuit pattern is magnified at high magnification and focused on a photoelectric converter by means of an objective lens, and the optical image is converted by the photoelectric converter into an electric signal which is used for judgement of defects. Simultaneously, the optical image of the circuit pattern at the same location is also projected on two photoelectric converters disposed at optically conjugate front and rear planes which are equi-distant from the former photoelectric converter. The output signals of the photoelectric converters are computed at a computing circuit in accordance with a predetermined evaluation function to determine amounts of blur, and under-focus, in-focus and over-focus are decided on the basis of the evaluation values.
This technique seems to be applicable to checking of patterns of LSIs but practically, an object to be checked is inherently different from an object to be photographed and various difficulties are encounted in checking of patterns.
More particularly, an LSI pattern standing for an object to be checked is very fine, generally in the order of 1 .mu.m, and the pattern is of a multi-layer structure, with the result that the quantities of light incident to the optical system greatly differ one location to another subject to focusing. Further, the image to be inspected must be magnified at high magnification and therefore it is greatly affected by vibrations. Moreover, since depending on ambient temperatures, the object to be checked deforms and accuracies of positioning required for an object carriage vary, it is needed that focusing be carried out continuously.